Metallization system for semiconductor devices, devices utilizing such metallization system and method for making devices and metallization system

ABSTRACT

A metallization system for metallurgically bonding a semiconductor die to metallic conducting slugs as terminals at the same time, and at the same temperature, that a surrounding glass sleeve is hermetically sealed to the conducting slugs for forming a zener diode, for example, is disclosed. The metallization system comprises a combination of aluminum, tin and palladium, for bonding to molybdenum, the aluminum being vapor deposited followed by a vapor co-deposition of aluminum and tin and further followed by a vapor deposited layer of palladium. 
     A method of making a device is disclosed wherein the die, two metal slugs of molybdenum and a preformed high temperature sealing glass are assembled together and subjected to a time-temperature cycle which includes a rapid rise to a high temperature at which metallurgical bonding of the die to the metal slugs and hermetic sealing of the glass to the metal slugs take place in a short time interval followed by rapid cooling of the assembly to a temperature slightly below the eutectic of aluminum and silicon.

This is a division of application Ser. No. 430,431, filed Jan. 3, 1974.

BACKGROUND OF THE INVENTION

This invention is related to that disclosed in application Ser. No.409,843, filed Oct. 26, 1973 in the names of Earl K. Davis and Kent W.Hansen and assigned to the same assignee as the subject invention.

This invention relates to semiconductive devices having increased powerand mechanical strength and to methods of making them and it is anobject of the invention to provide improved devices and methods of thisnature.

More particularly the inventon relates to a metallization system on asemiconductive chip, or die, and a method of forming it whereby thesemiconductive chip may be metallurgically bonded to the associatedterminals. In the case of devices such as zener diodes wherein asemiconductive chip, two terminal slugs and the glass sleeve areassembled together and subjected to a heating cycle for metallurgicallybonding the chip to the slugs and for hermetically sealing the slugs tothe glass sleeve, the metallurgical bond is formed at the same time asthe glass seal. This is achieved without destroying any of the junctionsor other properties of the semiconductive chip. The need for placing thechip and the slugs under stress is thus eliminated.

Throughout this patent application the terms semiconductive die, chip,device or the like are used interchangeably without intending anyrestriction unless specifically pointed out.

While the invention has specific application to diodes such as zenerdiodes, for example, it will be clear that the invention has applicationto any semiconductive device wherein a metallization system is utilizedfor bonding the device to the connecting terminals.

Metallization systems used with zener, and other diodes, have been ofthe chrome-silver-gold variety wherein subsequent bonding of the die tothe lead terminals or slugs has been with lead-tin solders, for example.Such bonding was carried out at relatively low temperatures and ofcourse required a surrounding glass sleeve which would seal to theterminals at about the same low temperatures. Such bonds while quitesatisfactory had relatively low strengths and low power carryingcapability. Accordingly it is a further object of the invention toprovide an improved metallization system of the nature indicatedproviding greater strength and increased power carrying capability.

Efforts to increase the power carrying capability and mechanicalstrength, according to the invention, utilizes metallurgical bonding ofthe die to the terminals. The metallization system for this purpose iscarried out at a substantially higher temperature. The latter requiresthe use of a surrounding glass sleeve which will seal to the terminalsat the same relatively high temperature. Moreover, the time-temperaturecycle of the metallization system and the glass sealing temperature hadto be such as to not damage or destroy the junction or other propertiesof the semiconductive device. Particularly this is important for lowvoltage alloy devices. The high temperature time interval thus has to beshort.

The aluminum-tin alloy system, according to the invention, has thedesired properties including that of enabling rapid cooling of thedevices from the glass sealing temperature without either sacrificingthe metallurgical bond quality of overstressing the die.

Accordingly, it is a further object of the invention to provide improveddevices and methods for achieving these desirable ends.

SUMMARY OF THE INVENTION

In carrying out the invention according to one form there is provided ametallization system for contacting and bonding to a semiconductorsurface comprising: a first layer of aluminum deposited on the surfaceof said semiconductor, and a second layer comprising a mixture ofaluminum and tin deposited on said layer of aluminum. More specificallya flash layer of palladium is deposited on said layer of aluminum andtin. More specifically still said first layer comprises aluminum ofabout 2000 to 3000A thickness, said second layer comprises 15,000 to25,000A thickness of a mixture of aluminum and tin in which mixture thealuminum comprises a minimum thickness of about 2000A, and said layer ofpalladium comprises a thickness of about 800 to 1000A.

According to a further form of the invention there is provided a methodfor forming a metallizaton system on a semiconductor surface forsubsequent bonding to an electrode comprising the steps of: preheatingsuch semiconductor surface to about 200° C, forming an aluminum-siliconalloy on such surface and a layer of aluminum thereover, removing saidpreheat, and forming a mixture of aluminum and tin on said layer ofaluminum, followed by a flash layer of palladium.

According to a still further form of the invention there is provided asemiconductor device having a metallurgical bond between thesemiconductor die and the terminals attached thereto comprising asemiconductor die, a metallization system on each side of said diecomprising an aluminum layer and a mixture layer of aluminum and tin,terminals comprising one of the group of metals of molybdenum, copper,nickel, silver, platinum, palladium, rhodium, tungsten and Dumet on eachside of said die and metallurgically bonded to said metallizationsystem, and a glass preform bonded to each of said metal slugs.

According to a still further form of the invention there is provided amethod of making a semiconductor device including a semiconductor chip,at least one terminal metallurgically bonded to said chip and a glasssleeve bonded to said terminal comprising the steps of: forming asemiconductor chip having a metallization system thereon comprising alayer of aluminum, a mixture layer of aluminum and tin on said aluminumlayer; providing at least one terminal consisting of one metal selectedfrom the group of molybdenum, copper, nickel, silver, platinum,palladium, rhodium, tungsten, Kovar and Dumet, disposing said terminaladjacent said semiconductor chip; providing a glass sleeve surroundingsaid terminal; subjecting the assembly of said chip, said terminal andsaid glass sleeve to a time-temperature cycle comprising an increase intemperature to about 725°-825° C in about 6-10 minutes followed by adecrease in temperature to slightly below the aluminum-silicon eutecticin a period of about 10-30 seconds and followed by a cool down to roomtemperature in about 5-7 minutes.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the invention reference should behad to the accompanying drawings in which,

FIG. 1 is a sectional view, somewhat diagrammatic, and on an enlargedscale, of a diode embodying the invention.

FIG. 2 is a sectional view on a larger scale of a portion of thestructure shown in FIG. 1 and illustrating its method of construction;

FIG. 3 is a sectional view on a larger scale of a portion of thestructure shown in FIGS. 1 and 2;

FIG. 4 is a block diagram illustrating steps in the formation of thesemiconductive device and its metallization system;

FIG. 5 is a diagrammatic view partially in section illustrating themethod of forming the metallization system;

FIG. 6 is a block diagram illustrating steps in the formation of thefinal device; and

FIG. 7 is a graph illustrating a time-temperature cycle according to theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the drawings there is shown in FIG. 1 a diode which, forexample, may be a zener diode comprising a semiconductive die, or chip10, a pair of metallic terminals or slugs 11 and 12 bonded at respectivesurfaces to the die 10, conductors 13 and 14 attached to and axiallyextending from respectively, terminals 11 and 12 and a glass sleeve 15bonded to the terminals 11 and 12. The bonds at the surfaces 16 and 17of the die 10 and the metallic slugs 11 and 12, respectively, aremetallurgical bonds in accordance with the invention, and the bondsformed between the interior surface 18 of the glass sleeve 16 and theadjacent surfaces of the metallic slugs 11 and 12 are high temperaturehermetic seals formed at the same time that the metallurgical bonds arebeing formed.

Referring to FIG. 2 there is diagrammatically shown an enlarged versionof the chip, or die, 10 and, in part, its manner of formation. Thus thedie 10 is shown between partially broken rectangles 19 and 21 whichrepresent additional and identical dice on each side of die 10. Thereare representative of a wafer represented by the broken line 22 whichwould include as many die 10 as may be formed within the wafer accordingto well understood principles. A large number of dice 10 are formed inwafer 22 and after all of the processing is carried out, as will bedescribed, individual dies 10, 19, 21, etc. are formed by laser scribingat the kerfs 23 and breaking into dice as is well understood. Theprocessing will be described with respect to the die 10 of FIG. 2, butit will be understood that the same processing will have been carriedout over the whole surface of the wafer 22.

As to the die 10 a body, or substrate 24, of silicon having N typedoping, for example, of suitable resistivity is selected and a P typeregion 25 is diffused therein to form a junction 26. If desired in aparticular case, the substrate 24 may be of P type doping and thediffusion 25 may be of N type doping. The diffusion 25 is performed inany well known manner such as by photoresist masking or the likefollowed by diffusion. Corresponding islands or P type regions 25 are ofcourse formed in all of the areas which will become the dice 19, 21,etc. After the diffusions have been made the upper surface 27 of thewafer, the wafer is covered with a layer 28 of silicon dioxide ofsuitable thickness. The silicon dioxide layer may be formed in any wellknown manner. After the layer of silicon dioxide 28 is formed, suitablewindows 29 are etched therein, one for each die in any suitable wellknown manner such as by the photoresist and etching techniques. Intoeach one of the windows 29 a metal system 31 is formed in good ohmiccontact with the P doped region 25 and a second metallization system 32identical to 31 is formed on the other side of the substrate 24.

The metallization system 31, 32, in accordance with the invention, andits method of formation, will be described in connection with FIGS. 2, 4and 5.

The metallization system comprises a combination of a layer of aluminumalloyed to the silicon and a combined layer of aluminum and tin formedover the first aluminum layer. A layer of palladium may be formed overthe combined aluminum and tin layer in order to prevent oxidation of thealuminum during storage of the dice before being utilized in theformation of devices.

Referring to FIG. 5 there is shown a vacuum chamber 33 comprising a base34 and an inverted bell jar, or the like, 35. The jar 35 may be sealedto the base 34 at the joint 36 in any suitable manner and any degree ofvacuum may be drawn through conduit 37 and valve 38. Interiorally of thechamber 33 there is a wafer 22 disposed on a suitable support 39 whichmay, for convenience, also be a heating arrangement of any well knownvariety for heating the wafer 22 up to a desired temperature, forexample, of about 200°C.

The wafer 22 in FIG. 5 has been processed as described and includes thelayer of silicon dioxide 28 and the windows 29. Also disposed insidechamber 33 are three containers 41, 42 and 43 containing respectivelyappropriate amounts of aluminum 44, tin 45 and palladium 46. Any wellknown type of heaters, electrical resistance, induction or the like 47,48 and 49 are provided for heating the containers 41, 42 and 43respectively.

After the wafer 22 has been heated to about 200°C, the aluminum 44 isheated to its boiling, or vaporization, temperature and a layer 51 ofaluminum is deposited. The aluminum is heated to its boiling point of2467°C and an equivalent thickness of about 2500A is deposited as shownin FIG. 4. The deposited aluminum forms a thin layer of aluminum-siliconeutectic alloy shown as 51A in FIG. 2 and a layer of pure aluminum shownas 51B in this Figure.

Throughout this specification, the symbol A means an Angstrom, a unit oflength equal to one ten billionth of a meter.

After the layer 51 of aluminum (51A and 51B) is deposited the heat isturned off from wafer 22 and it is permitted to cool down at whateverrate the apparatus provides. During the cooling process and immediatelyfollowing the initial aluminum deposition step the tin 45 is heated bythe heater 48 to its boiling, or vaporization, temperature of 2270°C.The aluminum remains at its boiling temperature. During this step thealuminum 44 and the tin co-deposit on the wafer to form the layer shownas 52 in FIGS. 2 and 5. The equivalent thickness of aluminum depositedduring this step is about 2500 and about 20,000A of tin are deposited.As a practical matter the amount of aluminum 44 and the amount of tin 45are selected ahead of time so that the vapor deposition of these twometals continues until the containers 41 and 42 are empty.

Following the co-deposition of aluminum and tin the palladium 46 isheated, by heater 49, to its boiling, or vaporization, temperature of2927°C to deposit a flash layer 53 of palladium about 800 to 1000Athick.

After the three layers of aluminum 51, aluminum-tin 52 and palladium 53are deposited the wafer is turned over and disposed on the support 39.The vacuum is again formed inside of chamber 33 and the same cycle ofdeposition of aluminum, tin and palladium as already described isapplied to give the layers, as shown, forming the metallization system32 of FIG. 2. Thus, there is a layer of aluminum 54 of about 2500A thickdeposited to form an aluminum-silicon eutectic alloy layer 54A and alayer of aluminum 54B on top of which there is co-deposited a matrix ofaluminum and tin 55 comprising an equivalent of an additional 2500A ofaluminum and about 20,000A of tin to give the co-deposit layer 55. Ontop of the aluminum-tin co-deposit layer 55 there is deposited a layerof palladium 56 of about 800 to 1000A thickness.

After the second metal system 32 has been completed, the wafer isremoved from the chamber 33 and is permitted to cool following which thewafer 22 is laser scribed at the kerfs 23 and broken into chips or diceto be subsequently formed into the diodes, for example, as will bedescribed.

While according to a preferred form of carrying out the inventionaluminum and tin are separately evaporated to form the co-deposit layer52, it will be understood that the layer 52 may be deposited from atin-aluminum alloy for example one in the proportions of 90 tin-10aluminum. The ratio of tin to aluminum may be varied for particularcircumstances and since these two metals boil at temperatures relativelyclose to each other, equal volumes of the metals may be deposited inabout the same time interval. The co-deposition layer should have aminimum thickness of 2000A in order to avoid subsequent peeling of thelayer. The initial layer of aluminum 51 should not be less than about2000A of equivalent thickness, and may have a thickness of between 2000and 3000A, the co-deposition layer 52 of tin and aluminum may have athickness of about 15,000 to 25,000A and the layer of palladium 31 mayhave a thickness of between 500 and 1000A.

After the dice have been formed as described a particular die 10,referring to FIG. 1, may be assembled in any suitable fixture as will bewell understood along with two metallic slugs 11 and 12 and asurrounding glass sleeve 15. This assembly may be passed through afurnace or sealer and subjected to the time-temperature cycleillustrated, for example, in FIG. 7 and the metallurgical bond betweenthe metal slugs and the die formed as well as the hermetic seal to theglass sleeve.

The metallic slugs 11 and 12 may be formed of any metal which will bewet by tin, which has the required properties of electrical conductivityand thermal expansion to be compatible with the glass of sleeve 15. Thusthe slugs 11 and 12 may be formed of molybdenum, copper, nickel, silver,platinum, palladium, rhodium tungsten, Dumet, Kover and perhaps others.The metal of the slugs in addition should not form brittle intermetalliccompounds with aluminum.

Forming a metallurgical bond between the die and the metallic slugsrequires that the bonding temperature be above the complete meltingpoint of the bonding metal, that is, not just about the eutectictemperature but the melting point of the highest melting point of thematerial there. Thus the aluminum in the subject invention must becompletely melted and the temperature must be at least 660°C, themelting point of aluminum. Preferably the bonding temperature should beat least 25°C above this melting point for example in the vicinity of685° to 700°C. In the actual case the percentages of aluminum and tinare not at the eutectic percentages so that the melting point is stillhigher than the eutectic and in the actual case temperatures in thevicinity of 725° to 825°C are feasible.

The foregoing also requires that the glass of the glass sleeve 15 softensufficiently to seal to the slugs 11 and 12 without melting to theextent of completely deforming. One composition of glass which has beenfound satisfactory but is not part of the subject invention is disclosedand claimed in the aforesaid co-pending application of Earl K. Davis andKent W. Hansen. The composition of said glass in range and weightpercent comprises the following and has been found to seal themolybdenum slugs without the presence of oxide thereon, in an atmosphereof nitrogen, at a temperature in the vinicity of 740° to 760° C in aperiod of time less than 1 minutes:

    ______________________________________                                                     Preferred   Range                                                SiO.sub.2      22            20-25                                            PbO            40            35-45                                            ZnO            10             8-12                                            Al.sub.2 O.sub.3                                                                              9             6-12                                            CdO             1            0-3                                              B.sub.2 O.sub.3                                                                              18            15-21                                                           100                                                            ______________________________________                                    

When the metal slugs 11 and 12 are formed of molybdenum they areprovided with a flash coating of nickel 57 (FIG. 3) of about 50 to 140micro inches in thickness and a flash coating of palladium 58 of about10 to 20 micro inches in thickness. In FIG. 3 the various layers areshown and will be described as though the metallurgical bonding andglass sealing cycle have been gone through. Whenever it is necessary toplate another metal to molybdenum, such, for example, as thealuminum-tin alloy, the coating of nickel 57 is desirable. A flashcoating of palladium or some othr noble metal is plated over the nickelto minimize oxidation of the nickel. In the event that the metallicterminal slugs 11 and 12 are of other metal such for example as Dumetalloy the flash coatings of nickel and palladium may not be necessary.

Referring to FIG. 6 there is shown a block diagram which represents theformation of the final diode device. This is shown, broadly, as a twostep method or process the first of which is assembling the die, contactslugs and glass preform, or sleeve, as may be visualized in FIG. 1. Thesecond step is subjecting the assembly to a time-temperature profileshown in FIG. 7 wherein the assembly is heated to about 725°-825°C andsubsequently cooled for performing the metallurgical bond of the die tothe slugs and a seal of the glass preform to the slugs.

Referring to FIG. 7 the time-temperature cycle according to one form ofthe invention comprises a rising temperature portion 59 terminating at aplateau region 61, an up spike region 62 terminating at a further holdor plateau 63, a downward spike region 64 and a cooling down region 65.

The heating of the assembled units may be carried out in a well knownbelt type furnace in which a temperature profile as shown in FIG. 7 ismaintained. The region 59 of heating from ambient to about 500°-550°Cwhich is below the aluminum-silicon eutectic temperature of 577°C inabout 5-7 minutes is not critical as to time. The time period can begreater or lesser than 5-7 minutes as may be appropriate. Thetemperature however should remain below the eutectic temperature inorder to prevent the diffusion of silicon from out of the junctionparticularly in alloy junction devices. The dwell time period 61, orplateau, shown as being from one to two minutes also is not critical andmay be longer or shorter. The plateau, or soak period, 61 isadvantageous in a belt type furnace in order that all parts includingthe devices and the boat containing them be up to temperature in orderthat the next period 62 may be carried out effectively. It is desirablethat the change from the temperature at the plateau 61 and thetemperature at the plateau or dwell 63 of 725°-825°C be fairly rapid. Ineffect, it should be a spike rise in temperature, shown for example as20 seconds to 1 minute, in order to avoid the diffusing out of siliconin the case of low voltage silicon alloy junction devices. When all ofthe parts start at the plateau temperature of 500°-550°C it is feasible,in a belt type furnace, to have the temperature follow along the line62. The plateau 63 temperature of 725°-825°C is selected to be above thealuminum-silicon eutectic temperature and to be above the melting pointof all of the aluminum present in the layers 51 (51A, 51B) and 52 inorder that a metallurgical bond may take place between the silicon ofthe die and the metal of the terminal slugs 11 and 12 for examplemolybdenum.

The time period of the dwell 63 is shown as 10-60 seconds and should beas short as necessary in order to avoid out diffusion of silicon fromlow voltage alloy devices.

The temperature at the plateau 63 and the dwell time thereat is suchthat the glass of the glass sleeve 15 softens sufficiently to flow intocontact with the adjacent surfaces 18 of the metal slugs 11 and 12 toform a sealing bond therewith. The temperature is sufficient however sothat the glass preform 15 does not lose its shape.

The downward temperature region 64 is in the nature of a spike down from725°-825°C to 500°-550°C which is below the aluminum-silicon eutectic.The time for the drop represented by line 64 is shown as 10-30 secondsand is relatively critical in that the time above the aluminum-siliconeutectic should be short to avoid out diffusion of aluminum from thejunction in alloy junction devices. The relatively rapid cooling at thisstage also serves to lock in some residual stresses which is ofadvantage in sharp knee type zener diodes. The rapid cool down of region64, however, does not overstress the die because the liquidus-solidustemperature range of aluminum-tin is fairly broad. The mixture ofaluminum and tin remains slushy until about 230°C.

The total time of the spike between the rise 62, the dwell 63 and thedrop 64 is relatively critical in that this total time should not begreater than that within which the out diffusion of silicon from thejunction in alloy junction devices becomes significant. The time rangeas shown for this region has been found to be such as to producecompletely satisfactory devices.

In other forms of sealers or furnaces, for example, the form known as aDAP sealer, the increase in temperature instead of following the regions61 and 62 may follow a line shown as the dotted line 66 where thetemperature goes directly from the 500°-550°C to the glass sealingtemperature at region 63. In this device similarly good metallurgicalbonds and good glass seals are formed.

The cooling down region 65 from 500°-550°C to ambient is shown as beingin the region of 5-7 minutes and is not critical. This can be longer orshorter as a particular furnace is programmed to operate.

The total time for the temperature cycling is shown in FIG. 7 as beingin the range of 10-20 minutes. This is a very short time compared toother ways of forming such devices and results in a very strongmetallurgical bond and glass seal as compared with known devices.Substantially increased power may be obtained without damage to thedevice and substantially increased strength is obtained.

The temperature at the dwell 63 also is as high as shown in order toproduce a glass seal in a relatively short time. A lower temperaturemight be utilized in some instances but this would require a longer timefor the sealing of the glass to take place and this time factor wouldnot necessarily be good in the instance of alloy junction type devices.

Referring to FIG. 3 there is shown a representation of what is believedmight be the case as to the nature of the metallic structure when themetallurgical bond is complete. Thus there is believed to exist at the Pregion 25 an Al-Si eutectic alloy layer plus possibly a trace of aternary alloy of Al-Sn-Si. Next to the Al-Sn eutectic there is believedto be a layer of Al-Sn eutectic with a Pd-Al eutectic matrix. Thepalladium (Pd) which comes from the flash layer 53 of palladium used forprotecting the aluminum from oxidation, is believed to take up anyexcess aluminum which might exist from the original aluminum deposition.Next to the Al-Sn eutectic is believed to be a layer of alpha Pd-Alalloy and/or alpha Pd-Al-Sn alloy. The very thin layer of palladium 53on the diode chip as well as the palladium layer 58 on the metallic slug11 are sufficiently thin as they do not significantly enter into theformation of the metallurgical bond.

The metallurgical bond formed between the chip and the terminal slugs bythe aluminum, tin and palladium metals, as described, and as believed toexist as represented in FIG. 3 has been found to be very strong. Forexample, when the slugs 11 and 12 are moved back and forth such as bymoving the lead conductors 13 and 14 back and forth until breakageoccurs, it has been found that the rupture or break occurs within thesilicon chip rather than in the metallic system of aluminum, tin andpalladium.

The aluminum-tin system according to the invention with the incidentaladdition of palladium has substantial advantages of strength andadditional power capability as compared with the well knownmetallization system of chrome-silver-gold.

The metallization system according to the invention is advantageous inseveral ways. Aluminum is a good electrical conductor and alloys wellwith silicon. It is in fact one of the dopant materials for forming theP type region of the diode and it forms a good ohmic contact withsilicon for good electrical properties. Tin is a good metal to use alongwith aluminum because, in its use in the combined layer of aluminum andtin, it prevents further penetration of the aluminum into the siliconparticularly during the spike cooling stage 64 so that the junction ofthe diode is not ruined. Thus it is possible to make low voltage alloyjunction devices using aluminum and silicon. The aluminum-tin systemminimizes cracking during rapid cooling because of the wide range ofliquidus-solidus temperature range of this system and it avoids thecracking which might be associated with an aluminum-silicon systemhaving a single point eutectic at 575°C.

When the aluminum, tin and palladium are deposited as described theresulting alloy by weight may have a composition of 80% tin, 13%aluminum and 7% palladium, varying perhaps to about 90% tin, 8 1/2%aluminum and 11/2% palladium.

Platinum and rhodium may be substituted for palladium if desired andwhen palladium is used on molybdenum slugs it acts as a fluxing agentfor removing oxide from the molybdenum.

While the invention has been described particularly in connection withsilicon as a semiconductive material it will be understood that this isexemplary only and that the metal system should function equally wellwith germanium and other semiconductive materials.

As has already been pointed out the thin or flash coating of palladiumover the co-deposition of aluminum and tin protects the aluminum fromoxidizing while the devices are in storage.

While tin has been described as being deposited by vaporization it willbe understood that this also is exemplary and that tin may be chemicallyor electrolytically deposited in appropriate situations. Correspondinglyafter aluminum has been deposited palladium can next be deposited andthen tin deposited chemically or electrochemically. In this case therewould be no co-deposition of aluminum and tin but the final compositionin the finished diode the metallic structure would be essentially thesame.

What is claimed is:
 1. A metallization system for contacting and bondingto a semiconductor surface comprising:a first layer of aluminumdeposited on the surface of said semiconductor, and a second layercomprising a mixture of aluminum and tin codeposited on said first layerof aluminum.
 2. The metallization system according to claim 1 includinga flash layer of a metal comprising one of the group comprisingpalladium, rhodium and platinum deposited on said second layer.
 3. Themetallization system according to claim 2 wherein the metal of saidgroup comprises palladium.
 4. The metallization system according toclaim 1 wherein said first layer comprises aluminum of about 200 to3000A thickness and said second layer comprises 15,000 to 25,000 Athickness of a mixture of tin and aluminum in which mixture the aluminumcomprises a minimum equivalent thickness of about 2000A.
 5. Themetallization system according to claim 1 wherein the aluminum of saidfirst and said second layers comprises an equivalent thickness of about5000A and the tin of said second layer comprises an equivalent thicknessof about 20,000A of tin.
 6. The metallization system according to claim4 including a palladium layer of about 800 to 1000A thickness over saidsecond layer.
 7. The metallization system according to claim 4 includinga rhodium layer of about 800 to 1000A thickness over said second layer.8. The metallization system according to claim 4 including a platinumlayer of about 800 to 1000A thickness over said second layer.
 9. Themetallization system according to claim 1 wherein said first layercomprises aluminum of about 2500A thickness, the mixture of said secondlayer comprises an equivalent thickness of about 2500A of aluminum andan equivalent thickness of about 20,000A of tin, and the metallizationsystem includes a palladium layer of about 800 to 1000A thicknessdeposited on said second layer.
 10. The metallization system accordingto claim 1 wherein the semiconductor comprises silicon.
 11. Themetallization system according to claim 1 wherein the semiconductorcomprises germanium.